net: emac: fix reset timeout with AR8035 phy

This patch fixes a problem where the AR8035 PHY can't be
detected on an Cisco Meraki MR24, if the ethernet cable is
not connected on boot.

Russell Senior provided steps to reproduce the issue:
|Disconnect ethernet cable, apply power, wait until device has booted,
|plug in ethernet, check for interfaces, no eth0 is listed.
|
|This appears to be a problem during probing of the AR8035 Phy chip.
|When ethernet has no link, the phy detection fails, and eth0 is not
|created. Plugging ethernet later has no effect, because there is no
|interface as far as the kernel is concerned. The relevant part of
|the boot log looks like this:
|this is the failing case:
|
|[    0.876611] /plb/opb/emac-rgmii@ef601500: input 0 in RGMII mode
|[    0.882532] /plb/opb/ethernet@ef600c00: reset timeout
|[    0.888546] /plb/opb/ethernet@ef600c00: can't find PHY!
|and the succeeding case:
|
|[    0.876672] /plb/opb/emac-rgmii@ef601500: input 0 in RGMII mode
|[    0.883952] eth0: EMAC-0 /plb/opb/ethernet@ef600c00, MAC 00:01:..
|[    0.890822] eth0: found Atheros 8035 Gigabit Ethernet PHY (0x01)

Based on the comment and the commit message of
commit 23fbb5a8 ("emac: Fix EMAC soft reset on 460EX/GT").
This is because the AR8035 PHY doesn't provide the TX Clock,
if the ethernet cable is not attached. This causes the reset
to timeout and the PHY detection code in emac_init_phy() is
unable to detect the AR8035 PHY. As a result, the emac driver
bails out early and the user left with no ethernet.

In order to stay compatible with existing configurations, the driver
tries the current reset approach at first. Only if the first attempt
timed out, it does perform one more retry with the clock temporarily
switched to the internal source for just the duration of the reset.

LEDE-Bug: #687 <https://bugs.lede-project.org/index.php?do=details&task_id=687

>

Cc: Chris Blake <chrisrblake93@gmail.com>
Reported-by: Russell Senior <russell@personaltelco.net>
Fixes: 23fbb5a8 ("emac: Fix EMAC soft reset on 460EX/GT")
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>